1. Field of the Invention
The present invention relates to an optical head for recording information into an optical disk and reproducing information from the optical disk and an optical disk unit having the optical head. In addition, the present invention relates to a method for eliminating an effect of a spherical aberration caused by the thickness error of a surface resin layer of an optical disk.
2. Description of the Related Art
In an optical head and an optical disk unit, in order to improve efficiency of utilizing light, there has been proposed a method and apparatus for reducing an effect of spherical aberration caused by the thickness error of a surface resin layer (light transmission layer) of an optical disk.
For example, in Jpn. Pat. Appln. KOKAI Publication No. 11-259906, there is disclosed an optical head 1 for emitting light from a light transmission layer 4 in a recording layer provided between a substrate 3 and the light transmission layer 4, wherein a collimator lens 13 and an actuator 14 for a collimator lens are provided, and the collimator lens 13 provided between a light source 10 and an objective lens 16 is moved so as to offset a spherical aberration caused by the thickness error of the light transmission layer 4.
In Jpn. Pat. Appln. KOKAI Publication No. 11-259906, when the collimator lens 13 is moved in order to correct the spherical aberration caused by the thickness error of the light transmission layer 4, no consideration is taken into the fact that a light amount on the objective lens 16 changes. This causes a problem in the case where light amount is set during reproduction in particular.
That is, when the collimator lens is moved in an optical axis direction in order to correct the spherical aberration caused by the thickness error of the light transmission layer, it is known that a quantity of light incident to the objective lens changes, and thus, the light amount emitted from the objective lens changes, making it difficult to ensure stable reproduction.